While driving at speeds above 55 mph, suddenly the transmission will "disengage" for lack of better terminology, i.e. no power to the rear wheels, and the engine RPM's will suddenly climb to 4000-5000 while my foot is still on the accelerator. Then a second later it resumes back to normal operating conditions. The problem gets worse the faster I go above 55. Driving around at slow speeds, even going up steep hills at 40 mph no problems.
Some other things going on that might be clues: there is a leak in the transmission so I'm always adding fluid. Also, the first time this issue presented itself while driving on the freeway, I pulled over and checked the fluid level and it was too high, about two inches higher than the full mark.
I had the car towed to my mechanic and they said it had very little fluid, which is confusing because the dipstick said otherwise. Any idea what's going on here? So they replaced the auxiliary pump seals, they said that's where the leak was, and added more fluid. Driving it again on the freeway yesterday for the first time since that episode, it happened again.
1987 300SDL

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1987 300SDL